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Commit a9668cd6 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

locking: Remove smp_mb__before_spinlock() 



Now that there are no users of smp_mb__before_spinlock() left, remove
it entirely.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent d89e588c

Documentation/memory-barriers.txt

+1 −4
Original line number Diff line number Diff line
@@ -1981,10 +1981,7 @@ for each construct. These operations all imply certain barriers: 
     ACQUIRE operation has completed.



     Memory operations issued before the ACQUIRE may be completed after

     the ACQUIRE operation has completed.  An smp_mb__before_spinlock(),

     combined with a following ACQUIRE, orders prior stores against

     subsequent loads and stores.  Note that this is weaker than smp_mb()!

     The smp_mb__before_spinlock() primitive is free on many architectures.

     the ACQUIRE operation has completed.



 (2) RELEASE operation implication:

Documentation/translations/ko_KR/memory-barriers.txt

+1 −4
Original line number Diff line number Diff line
@@ -1956,10 +1956,7 @@ MMIO 쓰기 배리어 
     뒤에 완료됩니다.



     ACQUIRE 앞에서 요청된 메모리 오퍼레이션은 ACQUIRE 오퍼레이션이 완료된 후에

     완료될 수 있습니다.  smp_mb__before_spinlock() 뒤에 ACQUIRE 가 실행되는

     코드 블록은 블록 앞의 스토어를 블록 뒤의 로드와 스토어에 대해 순서

     맞춥니다.  이건 smp_mb() 보다 완화된 것임을 기억하세요!  많은 아키텍쳐에서

     smp_mb__before_spinlock() 은 사실 아무일도 하지 않습니다.

     완료될 수 있습니다.



 (2) RELEASE 오퍼레이션의 영향:

arch/arm64/include/asm/spinlock.h

+0 −9
Original line number Diff line number Diff line
@@ -358,15 +358,6 @@ static inline int arch_read_trylock(arch_rwlock_t *rw) 
#define arch_read_relax(lock)	cpu_relax()

#define arch_write_relax(lock)	cpu_relax()



/*

 * Accesses appearing in program order before a spin_lock() operation

 * can be reordered with accesses inside the critical section, by virtue

 * of arch_spin_lock being constructed using acquire semantics.

 *

 * In cases where this is problematic (e.g. try_to_wake_up), an

 * smp_mb__before_spinlock() can restore the required ordering.

 */

#define smp_mb__before_spinlock()	smp_mb()

/* See include/linux/spinlock.h */

#define smp_mb__after_spinlock()	smp_mb()

arch/powerpc/include/asm/barrier.h

+0 −7
Original line number Diff line number Diff line
@@ -74,13 +74,6 @@ do { \ 
	___p1;								\

})



/*

 * This must resolve to hwsync on SMP for the context switch path.

 * See _switch, and core scheduler context switch memory ordering

 * comments.

 */

#define smp_mb__before_spinlock()   smp_mb()



#include <asm-generic/barrier.h>



#endif /* _ASM_POWERPC_BARRIER_H */

fs/userfaultfd.c

+11 −14
Original line number Diff line number Diff line
@@ -109,27 +109,24 @@ static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode, 
		goto out;

	WRITE_ONCE(uwq->waken, true);

	/*

	 * The implicit smp_mb__before_spinlock in try_to_wake_up()

	 * renders uwq->waken visible to other CPUs before the task is

	 * waken.

	 * The Program-Order guarantees provided by the scheduler

	 * ensure uwq->waken is visible before the task is woken.

	 */

	ret = wake_up_state(wq->private, mode);

	if (ret)

	if (ret) {

		/*

		 * Wake only once, autoremove behavior.

		 *

		 * After the effect of list_del_init is visible to the

		 * other CPUs, the waitqueue may disappear from under

		 * us, see the !list_empty_careful() in

		 * handle_userfault(). try_to_wake_up() has an

		 * implicit smp_mb__before_spinlock, and the

		 * wq->private is read before calling the extern

		 * function "wake_up_state" (which in turns calls

		 * try_to_wake_up). While the spin_lock;spin_unlock;

		 * wouldn't be enough, the smp_mb__before_spinlock is

		 * enough to avoid an explicit smp_mb() here.

		 * After the effect of list_del_init is visible to the other

		 * CPUs, the waitqueue may disappear from under us, see the

		 * !list_empty_careful() in handle_userfault().

		 *

		 * try_to_wake_up() has an implicit smp_mb(), and the

		 * wq->private is read before calling the extern function

		 * "wake_up_state" (which in turns calls try_to_wake_up).

		 */

		list_del_init(&wq->entry);

	}

out:

	return ret;

}